The invention relates to an amplifier circuit for amplifying electric signals, comprising controllable switching means for generating a block wave signal whose amplitude varies between first and second supply voltage values during operation, filter means for filtering said block wave signal so as to produce an output signal, which filter means comprise a self-inductance and a capacitance, means for providing a filter capacitance current proportional to the current through the filter capacitance, modulating means for pulse width modulation of the block wave signal by driving the switching means in response to an input signal to be amplified, and correction means for providing from a reference value derived from the input signal and an output signal value proportional to the output signal a correction signal for controlling the modulating means.
An amplifier circuit of this kind is disclosed in U.S. Pat. No. 5,606,289, in practice also known as a so-called class D amplifier.
In a class D amplifier circuit a block wave signal is generated which has a frequency which is much higher than the highest frequency of the input signal to be amplified. The pulse width ratio of this signal is modulated so that the average value of the block wave signal is proportional to the input signal. By applying the block wave signal to a low-pass filter or resonator circuit, with a cut-off frequency ranging between the highest signal frequency and the frequency of the block wave signal, an output signal is produced from which the switching frequency or block wave frequency and higher frequencies of the block wave signal have been removed. The output signal represents the average value of the block wave signal, and consequently of, the input signal, which is amplified, however, by an amplification factor which is determined by the electrical characteristics of the modulator, the correction signal, the supply source and the switching means. Usually, switching transistors such as MOSFET""s (Metallic Oxide Semiconductor Field Effect Transistors) are used as the switching means.
Linear amplifiers, such as class A and class AB amplifiers, for example, whose amplifier stage is essentially operated as a controllable series resistor, have a very low energetic efficiency, since a high heat dissipation occurs in the output stage when the amplifier is not driven to full load. An amplifier circuit comprising a switched output stage, on the other hand, such as the amplifier circuit according to the invention, only exhibits a small degree of heat dissipation, since the current through the output stage is zero when the switching means are off, and the voltage across the output stage is practically zero when the switching means are on. Switching amplifiers, or class D amplifiers, have a very high energetic efficiency,  greater than 90% in practice.
In practice a number of undesirable effects occur in switching amplifiers, which cause interferences of the ideal output signal. The interferences can be subdivided into internal errors and external errors.
The output impedance of the amplifier is mainly determined by the filter means for filtering out the block wave signal. This impedance is frequency dependent, and is for practical reasons approximately equal to the nominal load resistance at the end of the frequency band. Accordingly, interferences in the output signal due to external causes are hardly suppressed in the signal that is applied to the load. Furthermore, a load impedance-dependent frequency transfer will occur.
Furthermore, switching transistors, for example, have a limited response time, which is mainly caused by parasitic capacitances. Transistors connected in a so-called half-bridge circuit, wherein two switching transistors are arranged in series and the block wave signal is generated at the junction of the transistors, must never be xe2x80x9conxe2x80x9d simultaneously, since a current path from the positive supple terminal to the negative supply terminal will be formed in that case. The current that occurs during such a short circuit will undoubtedly cause damage to the switching transistors. For this reason a so-called xe2x80x9cdead timexe2x80x9d is maintained during the switching of the transistors, in order to ensure that at least one of the transistors will be xe2x80x9coffxe2x80x9d. Especially at small amplitudes this dead time causes a strong non-linearity in the signal transfer of a loaded amplifier circuit.
Finally, depending on the type of modulating means, interferences which are present on the supply voltage can be transferred to the output signal.
Both the output impedance caused by the output filter and the non-linearity resulting from the dead time and interferences on the supply voltage can be reduced as much as possible with the prior art switching amplifier circuits, provided they are adequately designed, by means of the correction signal in a closed loop feedback to the modulating means. It has been found, however, that the maximum suppression that can be realised is not adequate due to the output filter impedance/phase shift. Moreover, the stability condition may depend on the load and the supply voltage.
Accordingly, it is a first object of the invention to provide enhanced suppression of interferences in the output signal of a class D amplifier circuit caused by internal and external error sources by eliminating the influence of the output filter on the signal transfer characteristics of the amplifier circuit.
In accordance with the invention, this objective is accomplished by providing means for deriving a reference current from the input signal, wherein the correction means are arranged for providing the correction signal as a current correction signal from the reference current and the filter capacitance current.
The invention is based on the insight that an adequate differential correction is necessary in order to effect a quick correction of interferences in the output signal, to which end the invention advantageously makes use of the filter capacitance current, which is proportional to the derivative of the output signal voltage of the amplifier circuit, without the drawbacks of high-frequency noise and other interferences that occur when separate differentiating means are used for providing a D correction signal.
By placing the output filter within the control loop in accordance with the invention, the impedance of the output filter of an amplifier which is controlled in this manner will hardly result in an output impedance of the amplifier, if at all. A change in the load of the amplifier circuit will directly be detected and corrected, since the change in the filter capacitance current caused by said change in the load will immediately result in the generation of a current correction signal.
In a preferred embodiment of the invention the object is to minimise such interferences as quickly as possible, preferably within one switching period of the switching means. To this end the means for providing the filter capacitance current are wideband means, that is, comprising on average five times the signal bandwidth of the amplifier or higher.
Basically, there are two possibilities for measuring the filter capacitance current. In the case of direct measurement, a sensor or other electric component, such as a resistor, is connected in series with the filter capacitor. In the case of indirect measurement, the current is provided by a capacitor which is connected in parallel with the filter capacitance. The advantage of direct measurement is that the current through the filter capacitor can be represented with maximum accuracy and without any appreciable phase shift.
In a further embodiment of the invention, in order to provide a filter capacitance current which is maximally proportional to the current through the filter capacitance, the means for providing the filter capacitance current comprise a current transformer which is connected in series with the filter capacitance or with part thereof, which current transformer is built up of a core having a coaxial cable wound thereon, one conductor of which, for example the inner conductor, is connected in series with the filter capacitance, whilst a filter capacitance current proportional to the current through the filter capacitance is generated in the other conductor, the outer conductor of the coaxial cable.
The filter capacitance current and the reference current do not contain any information with respect to possible direct current (DC) components in the signal to be amplified. Accordingly, a preferred embodiment of the amplifier circuit according to the invention in the form of a voltage amplifier comprises a voltage correction signal in addition to a current correction signal.
In yet another embodiment of the amplifier circuit according to the invention, the correction signals are thus processed into one control signal for the modulating means in that the correction means comprise a first differential circuit for providing a first difference signal from the reference voltage and the output voltage signal, a second differential circuit for providing a second difference signal from the reference current and the filter capacitance current, a Proportional (P) or Proportionally Integrating (PI) control circuit including an input for the first difference signal, a control circuit including an input for processing the second difference signal by a factor (D) and a summing circuit for summing an output signal of the P or PI control circuit and an output signal of the D control circuit for controlling the modulating means.
This embodiment enables the control system to respond quickly and adequately, via the current feedback loop, to current variations on the output of the amplifier circuit, wherein the voltage feedback ensures that the system will follow the desired output level in the low-frequency range as well. In practice it has been found that the filter capacitance current responds to an insufficient degree in the low-frequency range, because the first order derivative is too small when signal frequencies are low, so that the voltage feedback loop must be designed to be operative at least in the low-frequency range of approximately  less than 500 Hz.
In the prior art a comparator circuit is generally used as the modulating means, the inputs being a triangular (or saw-toothed) voltage and the signal to be amplified with the possible addition of a correction signal. This technique is known as xe2x80x9csine-trianglexe2x80x9d modulation. In the case of sine-triangle modulation, the modulating means may cause relatively very narrow pulses when the amplifier is driven to a high degree of output, which narrow pulses may be harmful to semiconductor switching transistors. In addition to that, errors are introduced as a consequence of the dead time that is necessary. Errors of this type can be characterized as internal errors of the amplifier circuit. In addition to that, the output voltage of the sine-triangle modulator is proportional to the supply voltage being applied, which can be considered to be an external error.
Besides the sine-triangle modulation principle, also the xe2x80x9csigma-deltaxe2x80x9d modulation principle, which is not used very frequently in class D amplifiers, is suitable for use in an amplifier circuit according to the invention. In accordance with the sigma-delta modulation principle, the modulating means comprise a hysteresis control circuit. In contrast to sine-triangle modulators, the switching frequency of the switching means may vary under the influence of supply voltage and signal fluctuations with the sigma-delta modulation principle.
In a preferred embodiment of the invention, the modulating means comprise a hysteresis control circuit which functions to make it possible to vary the switching frequency of the switching means, which is not possible with the switching amplifiers which are known from the prior art. Basically, the switching frequency is free when using the hysteresis control circuit according to the invention, and it will vary with the supply voltage and the output voltage and output current without any further control. The switching frequency must be prevented from becoming too low in relation to the highest frequency of the input signal that is permissible, because this will result in an undesirably large switching ripple in the output signal.
In principle, interferences in the output signal of the amplifier circuit according to the invention are equalized within one, albeit extended, switching period. Consequently, an increased speed of response is obtained by dropping the principle of a fixed switching frequency, which makes it possible to realise a lower output impedance of the amplifier circuit.
In order to have the amplifier circuit operate at a desired average switching frequency, another embodiment of the invention has been configured with frequency control by providing the hysteresis control circuit with a control input for controlling its hysteresis window in response. The width of the hysteresis window determines the (average) switching frequency of the system, without affecting the characteristics as regards the elimination of interferences as described above, however.
Another advantage of using a hysteresis control circuit in the amplifier circuit according to the invention is that the integrating means required for the sigma-delta modulation principle, which integrate the difference between the output signal of the amplifier circuit and a desired value, are already implicitly present in the form of the filter inductance of the output filter, whose current is after all the integral of the difference between the block wave voltage of the switching means and the output voltage of the amplifier. The current through the filter inductance is partially available in the filter capacitor current, of course, a representation of which is according to the invention compared to a desired value of the reference current derived from the input signal and supplied to the hysteresis control circuit as a correction signal.
The advantage of the incorporation of the integrating means with the output filter that has to be present, is that interferences can be quickly minimized, preferably, as desired, within one switching period of the switching means.
Besides an embodiment in the form of a voltage amplifier, it is also possible to realise an embodiment in the form of an amplifier comprising a so-called current output. With such a current amplifier at least two of the three currents, viz. the filter self-inductance current, the filler capacitance current and/or the output signal current must be measured.
One embodiment of a current amplifier according to the invention comprises means for providing a filter self-inductance current proportional to the current through the filter self-inductance, means for providing an output current signal proportional to the output signal current, wherein the correction means comprise a first differential circuit for providing a first difference signal from the reference current and the output current signal, a control circuit including an input for the first difference signal and an input for an output voltage signal proportional to the output signal voltage, and a summing circuit for summing an output signal from the control circuit and a current value proportional to the filter self-inductance current for controlling the modulating means.
In the embodiment in question, the filter capacitance current is implicitly derived from the measurement of the filter self-inductance current and the output current signal.
The principle of the amplifier circuit according to the invention can be used both with a so-called half-bridge circuit, wherein a supply source having a positive and a negative voltage value relative to a zero point is available, and with a so-called fill bridge or H-bridge circuit both in the so-called (complementary mode controlled) xe2x80x9c2-levelxe2x80x9d mode and in xe2x80x9c3-levelxe2x80x9d mode. In the latter case an amplifier circuit built up of a first and a second amplifier circuit comprising a half-bridge circuit may be provided, wherein according to an embodiment of the invention the reference voltage and the reference current at the second amplifier circuit are processed in reverse phase compared to the first amplifier circuit.
In order to optimally eliminate interferences this amplifier circuit, which is connected as a full-bridge or H-bridge circuit, is according to the invention provided with a common hysteresis control circuit including a first and a second controllable hysteresis window for the first and the second amplifier circuit, wherein the hysteresis control circuit is controlled by means of a control signal comprising a differential term and a common term, wherein the differential term controls the desired phase difference between the first and the second amplifier circuit and the common term controls the average switching frequency of the first and the second amplifier circuit.
It is important thereby that the hysteresis control causes the phase of the pulses of the two bridge branches to be set exactly so that the double switching frequency is achieved on the output when the output signal is not equal to zero. A major advantage of this so-called xe2x80x9c3-levelxe2x80x9d mode is that there will be absolutely no switching ripple between the two outputs of the bridge branches when there is no input signal.
In circuits which are used in practice it is difficult to prevent the block wave-like output signal of the switching means comprising a common component in addition to the desired differential component. This is caused, among others, by small time differences in the modulation signals from the switching means. By suitably distributing the self-inductance of the output filter over the two bridge outputs and magnetically coupling the filter coils, it is possible to realise different inductances for the differential and the common (in-phase or xe2x80x9ccommon-modexe2x80x9d) signal components.
In another embodiment of the amplifier circuit according to the invention this has been realised in that the filter means for filtering the block wave signal of the full-bridge or H-bridge circuit comprise a self-inductance built up of an essentially 8-shaped core having a first and a second outer leg, each provided with a winding, and a central leg, which central leg has a higher magnetic resistance than the two outer legs, wherein the winding on the first outer leg is connected to the junction of the first and the second switching transistor and the winding on the second outer leg is connected to the junction of the third and the fourth switching transistor, in such a manner that an in-phase or common mode signal from the bridge circuit generates a magnetic field in the two outer legs of the core and in that a reverse phase signal from the bridge circuit generates a magnetic field through the central leg.
By using different capacitance values for the filter capacitance of the output filter, adapted to the realised inductance distribution, for the differential path and the common path as well, thus realising a low pass bandwidth for the common component in the output filter, this component can be further effectively suppressed.
In yet another embodiment of the invention a further improvement, in particular as regards the noise behaviour of the amplifier circuit, is achieved in that the differentiating means for forming the reference current and anti-aliasing input filter means are combined into one circuit on the input side, said circuit comprising a first differential amplifier including an input for connecting the input signal from the amplifier circuit and an output to which a low pass filter is connected, a second differential amplifier connected in cascade with the low pass filter as an integrator, which includes an output for supplying the reference voltage, and a third differential amplifier connected to the low pass filter for supplying the reference current.
The amplifier circuit according to the invention is excellently suitable for use as the output stage in an audio amplifier, but it can also be used advantageously in a power amplifier for precise measurement and control purposes, such as servo amplifiers having extremely high values for the product of power and bandwidth.